1. Field of the Invention
The present invention relates to electronic ballasts for gas discharge lamps, and more particularly, it concerns novel ballast arrangements and novel methods of recognizing gas discharge lamps by using fuzzy controllers.
2. Description of the Related Art
In the field of electronic ballasts, there are known ballasts which work with a positively controlled oscillator and are dimmable. For dimming a gas discharge lamp to be connected to the electronic ballast, the current flowing through the lamp is varied. This is achieved with the aid of the controlled oscillator by variation of the lamp current frequency. The gas discharge lamp is controlled via a series resonant circuit in its load circuit. If the frequency of the current delivered to the gas discharge lamp corresponds approximately to the resonance frequency of the series resonant circuit, the lamp is ignited. By displacing the current frequency away from the resonance frequency of the series oscillation circuit or towards the resonance frequency of the oscillation circuit, the current of the gas discharge lamp can be reduced or increased. For controlling the lamp current, the actual value of the momentary lamp current is measured and compared with a desired value. A correspondingly present current controller generates on the basis of these two values a setting value for the current. The lamp voltage sets itself in correspondence with the lamp characteristic.
Gas discharge lamps have a negative characteristic. This means that the lamp voltage falls if the lamp current increases. If the lamp is to be controlled to be brighter, the current must thus be controlled to be higher. However, because of the negative characteristic of the lamp, the fall off of the lamp voltage works against this.
For this reason it has been proposed to control not the lamp current, but rather the lamp power, i.e. the product of lamp current and lamp voltage. The setting of the lamp power is again effected by means of the frequency. The actual value of the lamp power is measured and compared with a desired value. In order to achieve a compensation of the control difference, i.e. the difference between the actual value and the desired value, the frequency is displaced from the resonance frequency of the series resonant circuit present in the load circuit of the lamp away from or towards the resonance frequency in dependence upon the sign of the control difference. Such ballasts have, however, the disadvantage that solely the lamp power can be monitored. Since only the product of lamp voltage and lamp current is controlled, it is not excluded that the electronic ballast might in some circumstances be controlled into an unstable or non-permitted region. It is thus, for example, conceivable that a limit value for the maximum permissable lamp power is complied with but a limit value for a maximum permissable lamp current is exceeded.